This invention relates to a method of and a system for controlling a hydraulic power system comprising an internal combustion engine receiving a supply of fuel from a fuel injection pump, and at least one variable displacement hydraulic pump driven by the internal combustion engine, such as a drive means for a hydraulic shovel.
Heretofore, in a hydraulic drive system wherein a plurality of variable displacement hydraulic pumps are driven by a single internal combustion engine for generating hydraulic power, such as a drive means for a hydraulic shovel, the three control systems have been available for distributing the output hosepower of the internal combustion engine to the hydraulic pumps; namely, an individual control system, a cross-sensing system, and an engine-speed sensing system.
In the individual control system when, for example, two hydraulic pumps are used, one-half the maximum output hosepower of the internal combustion engine is distributed to each hydraulic pump and the circuit pressure of each hydraulic pump is separately sensed to thereby control the discharge rate of the pump In this system, inconvenience is experienced in that when no load is applied to one of the hydraulic pumps, it is impossible for the other hydraulic pump to utilize the excess horsepower of the one hydraulic pump.
In the cross-sensing system when a load applied to one hydraulic pump is low, the other hydraulic pump utilizes part of the excess hosepower of the one hydraulic pump by allowing the circuit pressures of the two hydraulic pumps to be transmitted to each other This system is superior to the individual control system with respect to utilization of horsepower. However, the regulator for controlling the discharge rate of each pump is complex.
In the engine-speed sensing system, a drop in engine speed effects control of the discharge rate of each hydraulic pump and maximizes the utilization of horsepower. An example of this type of control system is disclosed in, for example, SAE PAPER 760687 (Electrohydraulic Control of Hydrostatic Transmission by T. P. Neal). This control method is superb in that the displacement volume of the hydraulic pump or its tilting angle is restricted by sensing a drop in engine speed.
In the above-mentioned control method of the prior art a mechanical all-speed governor is used for sensing a drop in engine speed. Because of this, when the engine speed undergoes a very small change, it does not follow that the change in speed represents a similar change in the engine operating condition in all the speed ranges. For example, when such small speed change occurs, the engine may tend to stall in a certain speed range, and when the same speed change occurs, the engine may continue to operate normally in other speed ranges. Moreover, mechanical all-speed governors may vary from one another due to production errors that might be committed in performing machining or due to changes with time. Thus, even in a specific speed range, a change in the engine operating condition indicated by a very small change in engine speed may finely vary from one mechanical governor to another and with elapsing of time. Thus, difficulties would be encountered in effecting adjustments of a set point for controlling a swash plate tilting in conformity with a change in engine speed. Even if an attempt is made to adjust the set point in such a manner that limitations are placed on the swash plate tilting by using a certain speed change as a reference, it would be impossible to maximize engine horsepower in other speed ranges than specific speed range.
Also, in the above-mentioned control method operating, characteristics undergo changes when a plurality of hydraulic actuators are connected to a single hydraulic pump as is the case with a hydraulic shovel. Because of this, the control method uses series compensation steps for preventing a delay in response to a variation in the discharge pressure of the pump. There are, however, limits to improvements in performance, so a problem arises with respect to the stability of the control system.
An object of the present invention in providing a hydraulic power system comprising an internal combustion engine receiving a supply of fuel from a fuel injection pump, and at least one variable displacement hydraulic pump driven by the internal combustion engine, a method of for controlling such hydraulic power system in which adjustments of the setting point are facilitated, whereby the engine horsepower can be effectively utilized to a maximum in all the speed ranges and good stability with respect to, for example, changes in the discharge pressure of the pump, for example can be achieved.
In accordance with the present invention a method of controlling a hydraulic power system comprising an internal combustion engine receiving a supply of fuel from a fuel injection pump, and at least one variable displacement hydraulic pump driven by the internal combustion engine is provided which includes the steps of obtaining a speed deviation from a difference between a target speed set by an accelerator operation of the internal combustion engine and an output speed of the engine; obtaining a fuel injection target value based on the engine speed deviation, with the fuel injection target value being in predetermined functional relationship to the deviation; controlling a fuel injection rate of the fuel injection pump based on the fuel injection target value; determining, at the same time, a displacement volume target value of the hydraulic pump based on at least the engine speed deviation and an actual discharge pressure of the hydraulic pump in such a manner that an input torque of the hydraulic pump decreases as the engine speed deviation increases; and controlling the displacement volume of the hydraulic pump based on the displacement volume target value.
Also, the invention provides a system for controlling a hydraulic power system comprising an internal combustion engine receiving a supply of fuel from a fuel injection pump, least one variable displacement hydraulic pump driven by the internal combustion engine, with the system comprising means for obtaining an engine speed deviation from the difference between a target speed set by an acceleration operation of the internal combustion engine and an output speed of the engine and means for obtaining a fuel injection target value based on with the engine speed deviation, said fuel injection target value being in predetermined functional relationship to the deviation. Means are provided for controlling a fuel injection rate of the fuel injection pump based on the fuel injection target value, and means are provided for determining, at the same time, a displacement volume target value of the hydraulic pump based on at least the speed deviation and an actual discharge pressure of the hydraulic pump in such a manner that an input torque of the hydraulic pump decreases as the speed deviation increases. Additionally means are provided for controlling the displacement volume of the hydraulic pump based on the displacement volume target value.